1. Field of the invention
The present invention relates to a zoom lens, especially to a projection zoom lens employed in a projection system, and more particularly relates to a zoom lens having a long back focal length.
2. Description of prior art
Conventionally, a zoom lens is designed for use in photography, therefore its back working distance, i.e., back focal length, is normally short but can meet the requirements of cameras. However, a conventional zoom lens cannot be applied to a projection device due to its short back focal length. For instance, referring to FIG. 1 of U.S. Pat. No. 4,190,323 a zoom lens is disclosed including a first lens group 10 and a second lens group 20, which has a viewing angle greater than 76.degree. and a zoom ratio of more than 2.3, but has a short back focal length.
A projection device, such as an LCD projector, requires a high resolution projection zoom lens due to the resolution required by LCD. Moreover, because of beam splitting in the device, the projection lens needs a long back working distance. Therefore, some zoom lens systems have been designed for use in projection devices in the prior art.
Referring to FIG. 2, Japanese Patent Laid-Open 6-130295 discloses a projection zoom lens which includes a first lens group 30 having a negative focal length and a second lens group 40 having a positive focal length. In such a projection zoom lens, the spherical aberration is within 0.5 mm, the astigmatism is within 0.5 mm, and the distortion is within 5%. However, the lens is provided with a back focal length longer than 99 cm. Although the back focal length can meet the requirement of a projection lens, the image quality is not desirable.
As described above, projection zoom lenses in the prior art are not suitable for use as projection lenses because the back focal length is too short or the image quality is not acceptable.